1. Field of the Invention
The present invention relates to an apparatus for controlling a fuel injection for an internal combustion engine, and more particularly to an apparatus having a sensor including a heater and arranged in an intake system downstream of a throttle valve, which sensor responds to a partial pressure of oxygen in air introduced into the engine, for detecting an amount of newly introduced air for controlling an engine control characteristic such as a fuel injection amount or an ignition timing.
2. Description of the Related Art
Known in the prior art is a device having a heater installed type oxygen sensor including a heater and responsive to an oxygen partial pressure of an intake air for detecting an amount of air newly introduced into an internal combustion engine for controlling an engine control parameter such as a fuel injection amount or an ignition timing. In this type of device, the oxygen sensor comprises a body formed of a solid electrolyte, such as zirconia, platinum electrodes arranged on both sides of the body, and a diffusing layer of a porous ceramic material on one side of the body and covering the platinum electrode on that side. Here, O.sub.2 particles are passed through the diffusing layer by a pumping operation at a controlled rate so that a limited electric current is obtained. The value of the limited electric current obtained when a predetermined constant voltage is applied across the electrodes on the electrolyte body is proportional to the value of the oxygen partial pressure in the total pressure of the intake air, and as a result, an amount of air newly introduced into the engine, which is proportional to the oxygen partial pressure, can be detected. An engine control of an engine parameter such as a fuel injection amount or an ignition timing is carried out in accordance with the amount of newly introduced air detected by the oxygen partial pressure. This type of control system including the oxygen partial pressure sensor for detecting the amount of newly introduced air is advantageous in that the arrangement of the sensor in the intake system incurs only a very limited increase of an intake resistance, and in that a precise control of the air-fuel ratio can be made since the amount of newly introduced air is itself directly detected. It should be noted that, in most conventional systems, the amount of air is not directly detected and the intake air pressure is indirectly detected as a measure of the amount of newly introduced air.
To obtain a high degree of precision in the detection of the oxygen partial pressure by this type of sensor, the temperature of the sensor must be controlled to a predetermined value, and accordingly, the sensor is provided with an electric heater device and a device for controlling the electric current or voltage in the heater. Nevertheless, when the engine is started, the temperature of the sensor is not rapidly increased even if the heater is operated, and thus a partial pressure of the oxygen can not be correctly detected. As a result, a control of the fuel injection amount or ignition timing based on an incorrect value of the oxygen partial pressure is carried out, and thus a precise control of the air-fuel ratio or ignition timing to a target value can not be made.